1. Field of the Invention
The present invention relates to a method for manufacturing a solid-state image sensor.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2007-201267 discloses a pixel structure in which the depth of a semiconductor layer constituting a photodiode changes depending on the color. According to Japanese Patent Laid-Open No. 2007-201267, this arrangement can effectively reduce a dark current and suppress generation of a white spot without adversely affecting an accumulated charge clear operation by an overflow drain. Japanese Patent Laid-Open No. 2008-270299 discloses an arrangement in which a contact plug for fixing the potential of a well is arranged in only some of a plurality of pixels.
In the arrangements described in Japanese Patent Laid-Open Nos. 2007-201267 and 2008-270299, the saturated charge quantity may differ between pixels having different structures. This is because the net quantity (difference between the quantity of doping of a donor and the quantity of doping of an acceptor) of doping impurities in a region constituting a charge accumulation portion differs between pixels having different structures. More specifically, in the arrangement described in Japanese Patent Laid-Open No. 2007-201267, the saturated charge quantity is smaller for a pixel in which the depth of a semiconductor layer constituting a photodiode is shallow. In the arrangement described in Japanese Patent Laid-Open No. 2008-270299, the saturated charge quantity of a charge accumulation portion arranged near a region in which a contact plug is connected is smaller than that of the remaining charge accumulation portions. This is because the formation of a well contact region in which the contact plug is connected influences the impurity density distribution of the charge accumulation portion.
If a pixel having a large saturated charge quantity and a pixel having a small saturated charge quantity exist in a pixel array, charges may move to an adjacent pixel when, for example, a pixel with a small saturated charge quantity reaches the saturated charge quantity at a high illuminance. This generates a color mixture, impairing the color reproducibility.
The present invention provides a technique advantageous to reduce a difference in saturated charge quantity between pixels.